Searchlight signals are used along railway tracks to convey information to the train driver, they can display three colours, usually red, yellow or green.
They are traditionally composed of a single incandescent lamp bulb and three coloured filters to be placed selectively in front of the bulb. A mechanism selects the proper filter to be used. This system is controlled by an external control voltage fed by a controller. In the absence of any control voltage, the red filter remains in front of the bulb, displaying a red aspect. If the control voltage is positive (12V), the yellow filter is moved in front of the bulb and a yellow aspect is displayed. If the control voltage is negative (−12V), the green lens is placed in front of the bulb, displaying a green aspect. Terminals are also providing feedback information to the controller regarding which colour is currently displayed or not displayed.
The maintenance costs of such a searchlight signal are high due to the relatively short life of the incandescent lamps and the required periodic maintenance and calibration of the mechanism.
Railroads are moving towards replacing incandescent bulbs with Light-Emitting Diodes (LEDs) as signals, resulting in increased visibility and longer life.
However there are two major issues with replacing incandescent bulbs with LEDs in searchlight signals. First, the existing cables buried in the ground between the controller and the searchlight signal may not support the wire gauge necessary to interface to the LED signal since the searchlight control and feedback interfaces are low current interfaces. Second, the logic in the controller must be changed to properly interface to the LED signal since the LED signal does not have the same electrical interface to the controller as the searchlight mechanism and incandescent bulb. Both of these modifications are extremely costly.
The document U.S. Pat. No. 6,642,666 B1 describes a searchlight signal comprising a colour selection interface to select the display aspect, a plurality of LEDs and a switching power supply.
However, it requires many components (diodes, resistors, relays, etc.) to electrically steer the current to the appropriate LED. This interface is costly and difficult to design to ensure safe design principles: by default, in case of problem, the signal must be red or not lit.